The skin consists of epidermis, dermis and hypodermis, of which the epidermis is what is called epithelium and derives from ectoderm, while the dermis and the hypodermis are connective tissues deriving from mesoderm (mesenchyma). The epidermis is stratified by germinative layer (basal layer), prickle-cell layer, granular layer, and so on. The cells which constitute epidermis are roughly divided into epidermal cells called malpighian cells or keratinocytes, and melanocytes having branch-like projections. The keratinocyte is characterized by keratinization and the melanocyte is characterized by melanin production. The keratinocytes are main cells constituting the entire epidermis, and the melanocytes exist mainly in the germinative layer of the epidermis.
The outermost part of the epidermis is called corneous layer and is a sediment of scale-shaped dead cells, which contains keratin in a large amount. The regeneration of epitheliocytes involves the steps of keratinocyte regeneration in the innermost germinative layer, moving thereof to the upper end of the corneous layer, and scaling off, which undergo in about 15 to 30 days' cycles.
While melanocytes produce melanin, pigment amount does not depend on the number of the melanocytes, but on pigment production and pigment distribution of the melanocytes. For example, there is no difference in distribution density of melanocytes between races such as blacks and whites, but if the produced melanin granules gather at one place, the color of the skin becomes fair, and if small granules widely spread, if becomes dark.
Both morphologically and functionally, the epidermis can be considered a symbiotic tissue of two independent cell elements --keratinocyte and melanocyte.
As the growth factor for epitheliocytes, EGF (epidermal growth factor) has been under investigations in terms of clinical effects [Nanney, L. B., J. Invest, Dermatol., 95, 624-629 (1990)], and large scale production (Earth Chemical Company, Japanese Patent Unexamined Publication No. 104293/1990), aiming at its practical use. EGF is a polypeptide consisting of 53 amino acids, which has a molecular weight of about 6 kD (kilodalton) and is known to have a cell growth accelerating effect on epitheliocytes, fibroblasts and vascular endothelial cells.
Although various effects have been expected of EGF as a medical agent, such as a vulnerary agent, a peptic ulcer-treating agent, a carcinostatic or an agent for artificial skin, it has not been put to practical use. The characteristic action and effect of the EGF is that is proliferates fibloblasts as well as epitheliocytes, and for this effect, it is suitable for the treatment of injury which has reached connective tissues, but it is hardly most suited for the treatment of injury and dermoulcer where only epidermis is the lesion. In view of canceration inducing action, EGF should be cautiously put to practical use.
Also, FGF (fibloblast growth factor) which is being developed as a vulneary agent like EGF mainly accelerates growth of fibloblasts, and selective proliferation of epitheliocytes only is unattainable. Although TGF-.alpha. which is structurally analogous to EGF also possesses epitheliocyte growth accelerating activity, administration thereof to humans or animals is considered to be difficult due to a grave defect that it also possesses cancerating activity as the name, transforming growth factor, i.e. canceration accelerating factor, indicates.